Structure and physiology of the development of roots from microcalli of leaf protoplasts of Ri-transformed and wild type Nicotiana plumbaginifolia

Data are presented of a comparative study on the structure and the physiology of the formation of single roots from microcalli, derived from leaf protoplasts of a wild-type clone (Npwt) and an Ritransformed clone (Np99) of Nicotiana plumbaginifolia, during culture in jV-KM medium containing naphthyl acetic acid (NAA) and zeatin. Optimum concentrations for root induction and development were found to be 0-1 to 0-3 mg I-1 NAA for Npwt and 0-3 to 10 mg 1 1 NAA for Np99, both at 01 mg I-1 zeatin. Root formation in Np99 occurred at higher frequency and reproducibility than in Npwt. The rate of development and the morphology of the roots were equal however for both clones. Subculture of rooted microcalli on hormonefree MS medium resulted in abundant hairy-root growth and shoot formation in Np99, whereas only occasional shoot formation occurred in Npwt. Structural research on root initiation and development, carried out on Np99, revealed the presence of a small, distinct group of cytoplasm-rich cells, at approximately 10 days after protoplast culture. This group of cells, which seemed to have a single-cell origin, showed unipolar growth, resulting in the formation of a root meristem, followed by the emergence of the young root from the microcallus, several days later. Ultrastructural studies showed the presence of osmiophilic droplets and amyloplast development in the tip region of the young roots.